Peloroplites: That’s One Big Ankylosaur

The "monstrous heavy one" was stout, armored and may have supported huge spikes on its neck and shoulders

The nearly-complete hindlimb of the armored dinosaur Peloroplites on display at the CEU Prehistoric Museum. Photo by author.

I love small dinosaur museums with a focus on local discoveries. You can go to almost any museum and see a Tyrannosaurus tooth or skull cast—there must be some unwritten rule among curators that the great tyrant must make a least a cameo in any dinosaur exhibit—but small museums often pay tribute to species that are not as widely known. The College of Eastern Utah’s Prehistoric Museum in Price, Utah is one such place. Upstairs, in a small gallery devoted to the region’s armored dinosaurs, are the robust, red and gray bones from a local giant described just a few years ago. This dinosaur is called Peloroplites cedrimontanus.

Peloroplites is just one of the latest ankylosaurs to be found in eastern Utah. The Cedar Mountain Formation, representing Early Cretaceous ecosystems from about 127 million to 98 million years ago, contains the fossils of at least six different species of ankylosaur (though they are distributed over this time period and did not all coexist with each other). Peloroplites was one of the biggest—its name means “monstrous heavy one”—and was found in strata dating to about 112 million years ago.

Though Peloroplites was a sturdy dinosaur, the fossilization process was not very kind to the specimen paleontologists Ken Carpenter, Jeff Bartlett, John Bird and Reese Barrick described in 2008. The new dinosaur was represented by a partial skull, portions of the arms and legs, parts of the hip, various vertebrae and a few other scraps. Altogether, though, the fossils indicated that Peloroplites was a relatively long-snouted ankylosaur that may have grown to lengths in excess of 16 feet. Compared to other ankylosaurs found in the area, Peloroplites was quite large, and the stout leg bones I saw at the CEU museum attest to the animal’s size.

Frustratingly, the heavy coat of bony armor this dinosaur would have sported is known only from a few small pieces, but Carpenter and co-authors were able to determine that Peloroplites belonged to a specific subgroup of ankylosaurs called nodosaurids. These dinosaurs lacked the heavy tail clubs often associated with ankylosaurs and, instead, often supported huge spikes on their necks and shoulders. Peloroplites may have been similarly outfitted. More than that, Peloroplites may have resembled an equally large ankylosaur that lived around the same time in prehistoric Montana called Sauropelta—so much so that Carpenter and co-authors suggested that some bones previously attributed to Sauropelta might turn out to belong to Peloroplites.

Just why Peloroplites and so many other ankylosaurs—some of which are still awaiting description—thrived in prehistoric eastern Utah is unclear. Given the numberof dinosaurs identified from the Cedar Mountain Formation in the past few years alone, paleontologists are still putting together a picture of what life was like during the days of the Early Cretaceous in prehistoric Utah. Vestiges of Jurassic ecosystems mixed with lineages that would become dominant later during the Cretaceous during this portion of geologic time, and investigations into how the aggregations of dinosaurs in the Cedar Mountain Formation evolved are ongoing.

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